Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using Freon type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, perishable items, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe, server and workstation computers. Though vapor compression cooling can be very efficient, it does require significant moving hardware. Vapor compression cooling systems, at a minimum, include a compressor, a condenser, an evaporator, and related coolant transfer plumbing. As a result of the complexity and associated high cost, vapor compression cooling has not found material acceptance in small cooling applications, such as personal computers, integrated circuits, etc.
The fact that CMOS logic can operate significantly faster as the temperature decreases has been well known for many years. For example, when CMOS logic devices are operated at -50.degree. C., their performance is improved by 50 percent over room temperature operation. Liquid nitrogen operating temperatures, in the range of -196.degree. C., have shown 200 percent performance improvements. Similar benefits have been shown to accrue for integrated circuit wiring, where metal wiring resistance decreases by a factor of 2 for integrated circuits operated at -50.degree. C. in comparison to room temperature operation. These performance improvements rival the recent technological breakthrough of using copper wiring in integrated circuits to reduce interconnect resistance and thereby effectively increase the operating frequencies attainable. Thus, sub-ambient temperature operation of integrated circuit logic devices, such as field effect transistors, as well as interconnect wiring can improve integrated circuit performance. This performance enhancement then poses the question of how to accomplish such cooling in the confines of the ever decreasing size and materially shrinking cost environment of microelectronics.
FIG. 1 schematically depicts a conventional Peltier type thermoelectric element (TE) 1 with DC power supply 2 creating the electric field across TE 1 while at a load current 3. The desired heat transfer is from cold sink 4, at temperature T.sub.cold, to hot sink 6, at temperature T.sub.hot. As indicated in the equation of FIG. 1, the net heat energy transported is composed of three elements, the first representing the Peltier effect (thermoelectric) contribution, the second defining negative Joule heating effects, and the third defining negative conductivity effects. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T.sub.cold) and the current being applied. The Joule heating component reflects that roughly half the Joule heating goes to the cold sink and remainder to the hot sink. Lastly, the negative component attributable to thermal conduction represents the heat flow through the Peltier device, as defined by the thermal conductivity of the Peltier device, from the hot sink to the cold sink. See equation (1). EQU q=.alpha.T.sub.cold I-(1/2)I.sup.2 R-K.DELTA.T (1)
There presently exists a need for thin film implementations and minitarization of thermoelectric coolers. Thin film implementations and minitarization of thermoelectric coolers would provide high cooling flux scaling with the smaller geometries to provide cooling in the range of 50 W/cm.sup.2 to 100 W/cm.sup.2 with high entropy gradients and lower thermal conductivities. Use of thin film implementations would yield higher reliability in the order of MTBF (mean time between failures) of greater than 10.sup.6 hours, lower cost in the order of less than 10.cent./W and ease of constructing multistage configurations wherein menocoolers can be operated in parallel for large cooling capacity and high efficiency.
Thus, materials and processes presently used in fabricating current thermoelectric coolers limit the use, minitarization, scalability and efficiency of thermoelectric coolers for providing cooling creating a need to overcome these limitations.